The invention relates to a hydraulic vehicle brake and to a method for operating same.
From EP 0 729 871 A1 a hydraulic vehicle, brake is known, wherein a friction pad can be pressed against a brake disk by means of a brake piston using hydraulic fluid, in order to brake its rotation. For this purpose, hydraulic fluid is fed under pressure in a conventional, manner into a hydraulic chamber, the boundary wall of which is formed by the brake piston. The pressure in the hydraulic chamber, then leads to a displacement of the brake piston and thus of the friction pad towards the brake disk. In order to enable this known vehicle brake to be used not only as a service brake, but also as a hand brakexe2x80x94also referred to as parking brakexe2x80x94, it comprises a spindle/nut arrangement for mechanically blocking the brake piston in a condition in which the friction pad is in engagement with the brake disk.
The invention is based on the object to provide a hydraulic vehicle brake which can be used both as a service brake and a parking brake, and the construction of which is more compact than that of the vehicle brake known from EP 0 729 871 A1. In addition, the hydraulic vehicle brake to be provided must be adaptable to various vehicle models in an economical manner.
According to the invention, this object is solved by a hydraulic vehicle brake which comprises the features of the present invention.
The electric motor which rotationally drives the spindle according to the invention is arranged in such a manner that its output shaft which extends at a distance and parallel to the longitudinal center axis of the brake piston exits the electric motor at the side facing away from the brake piston. This arrangement is space saving, in particular with respect to the length of the construction. The reduction gear connected between the electric motor and the spindle with its reduction in the order of 200:1 enables the employment of a small space saving electric motor.
The electric motor and the reduction gear which is coupled with same are designed as a standardized subassembly which can be handled independently and can be combined with various vehicle brakes of one type. Thereby, the production quantities of such a motor/gear unit increase, which has a favorable effect on the unit cost. Moreover, the motor/gear unit designed as an independent assembly enables an attachment at the brake housing in a simple manner in any desired angular position so that spatial restrictions at the installation site of the brake can easily be complied with by a correspondingly different positioning of the subassembly.
The reduction gear is advantageously designed as a planetary gear, a harmonic drive gear, an eccentric disk gear, or a swash plate gear, because these gear types are compact and, at a small weight, have a good efficiency. The reduction gear can be designed to be single-stage, but also multistage, in particular, two-stage. A two-stage gear is particularly advantageous with respect to saving installation space, one stage of which is formed by one of the previously mentioned gear types with a reduction in the range of 50:1, upstream or downstream of which a second stage with a reduction in the range of 4:1 is connected, so that an overall reduction in the range of 200:1 is achieved. In a multistage reduction gear, in particular in a two-stage reduction gear, preferably the stage with the lower reduction is designed as a simple spur gear or, more advantageously, as a belt gear, in particular, as a toothed-belt gear. The use of a belt gear or a toothed-belt gear results in a particularly good noise decoupling between the driving electric motor and the remaining parts of the vehicle brake, and thus to a comfortable, low-noise operation.
In all embodiments of the vehicle brake according to the invention the spindle/nut arrangement is advantageously designed to be self-locking, generally by a suitable selection of the pitch of a thread which connects the nut with the spindle. A self-locking spindle/nut arrangement allows to dispense with a locking device after the vehicle brake has been locked by rotating the spindle.
The spindle/nut arrangement is preferably arranged in the hydraulic chamber. This leads to a still more compact construction and, moreover, ensures the lubrication of the in spindle/nut arrangement. According to a further development of this embodiment, the brake piston is designed as a hollow piston which is open at one side so that the spindle/nut arrangement is at least partially accommodated in the brake piston in a space-saving manner.
In preferred embodiments of the vehicle brake according to the invention, the translational movement of the nut, which is caused by the rotation of the spindle, is sensed by means of a sensor. In this manner, a predetermined small distance between the brake piston and the nut, and thus an optimum function of the hand brake or the emergency brake, respectively, can be ensured. In practical use with the hand brake or the emergency brake, respectively, not actuated, a distance between the nut and the brake piston of approx. 0.5 mm is desired. A distance between the nut and the brake piston, which remains constant at all times, makes it possible, on the one hand, to achieve the desired maximum clamping force of the hand brake within a short period of time, i.e. within the range of approx. 1 second, and, on the other hand, allows the brake piston to return after an actuation in accordance with the so-called xe2x80x9crollbackxe2x80x9d principle, and to not contact the nut prematurely, which might result in a residual sliding moment between the friction member and the rotor to be braked. Those with skill in the art understand the xe2x80x9crollbackxe2x80x9d principle as the resetting of the brake piston following a braking operation, which is caused by the resetting force of the elastomeric seal sealing the brake piston, which seal has been elastically deformed during the braking operation. The mentioned sensor for sensing the translational movement of the nut can, for example, be a sensor which directly senses the travel of the nut. For this purpose resistive sensors, inductive sensors, or capacitive sensors as well, are suitable. In order to improve the brake control, in addition to the sensor sensing the nut""s travel, a force sensor sensing the clamping force can be provided which, for example, can be arranged in the or at the friction member, or in the brake piston as well.
The sensor sensing the translational movement of the nut can alternatively be a sensor which measures the revolutions of the output shaft of the electric motor driving the spindle. Instead of measuring the revolutions of the output shaft of the electric motor itself, it is also possible to measure the revolutions of a gearwheel in the downstream reduction gear. For measuring the revolutions, a Hall sensor is particularly suited, which provides a square wave output signal. If such a Hall sensor is aligned, for example, with the teeth of a gearwheel which is coupled with the output shaft of the electric motor, one revolution of the output shaft will correspond to a certain number of square wave pulses, i.e. to the number of teeth of the gearwheel. With the high resolution of the rotational movement of the output shaft of the electric motor, which is achieved in this manner, the translational movement of the nut, which is caused by the rotation of the spindle, can be sensed with a very high accuracy since there is a fixed correlation given by the existing reduction between the revolutions of the electric motor, on the one hand, and the translational movement of the nut, on the other hand. Moreover, a Hall sensor is very well suited for an integration into the already mentioned unit consisting of the electric motor and the reduction gear so that long electrical connections which are therefore potentially more susceptible to failure between the Hall sensor and the electric motor are avoided. A control unit controlling the functional sequences of the vehicle brake according to the invention is advantageously also arranged in the motor/gear unit.
Due to the fact that the clamping force of the brake can be deducted from the time characteristics of the current consumption of the electric motor, the further above mentioned travel sensor or the force sensor as well are no longer necessary, if a Hall sensor or another sensor is provided which directly or indirectly measures the revolutions of the output shaft of the electric motor. This means that it is no longer required to install sensors in the vehicle brake housing itself so that electrical connections are no longer necessary between the vehicle brake proper and the unit consisting of the electric motor and the reduction gear. Thus, merely the mechanical connection between the spindle and the reduction gear exists between the motor/gear unit and the vehicle brake proper.
The vehicle brake according to the invention is further advantageous in that the spindle/nut arrangement which cooperates with the brake piston cannot only be used as a blocking means, but that the spindle/nut arrangement furthermore provides a mechanical emergency actuation by means of which the friction pad can be pressed against the rotating rotor of the brake, e.g. in the case of a failure of the hydraulic actuation. The vehicle brake according to the invention can be designed as a disk brake or a drum brake as well.
In the following, an advantageous method will be described which allows the use of a generic vehicle brake, in which the spindle is driven by means of an electric motor, as a hand brake. For this purpose, the spindle is first rotationally driven in a first sense of rotation by the electric motor in such a manner that the nut of the spindle/nut arrangement is moved towards the brake piston. The characteristics of speed and current consumption of the electric motor are measured as a function of time. If a sudden speed drop and an increase in current consumption corresponding therewith occur, which means that the nut has come into contact with the brake piston, the values of speed and/or current consumption applicable before the speed drop or the increase in current consumption, respectively, are stored in a memory as reference values. The spindle is further rotationally driven in the first sense of rotation until the continued measurement of the current consumption shows that a previously defined maximum value of the current consumption has been reached which corresponds to the desired clamping force. As soon as this value is reached the electric motor will be switched off, and the vehicle brake fulfils its blocking function.
If the hand brake is to be released again, the spindle is driven rotationally counter to the first sense of rotation, and the momentary speed and/or the momentary current consumption of the electric motor are compared with the corresponding reference value which has previously been stored. The point of time in which the momentary speed and/or the momentary current consumption is identical with the corresponding reference value, which is equivalent to the nut clearing the brake piston, is sensed, and the spindle is further rotationally driven counter to the first sense of rotation in order to set a desired distance between the nut and the brake piston, which is mandatory for a satisfactory function of the vehicle brake.
The setting of a desired distance between the nut and the brake piston can be effected, for example, in such a manner that the spindle from the point of time onward in which the momentary speed and/or the momentary current consumption is identical with the corresponding reference value is rotationally driven for a specified period of time counter to the first sense of rotation, with the period of time being specified under consideration of the existing speed reduction of the electric motor in such a manner that the desired distance is exactly achieved. Alternatively, the continued rotational drive of the spindle counter to the first sense of rotation can be performed from said point of time onward until a certain number of revolution count pulses is reached, which for example is sensed by the above described Hall sensor. The certain number of revolution count pulses, in turn, is specified in such a manner that, at a given reduction of the speed of the electric motor, it results in the desired distance between the nut and the brake piston.
It is of particular advantage if, in the previously explained method, both the speed and the current consumption of the electric motor are utilized for controlling because this results in a certain redundancy. In a properly functioning brake, the break points in the time characteristics of the speed and the current consumption of the electric motor each must occur nearly simultaneously. Potential malfunctions can thus be detected early by monitoring the two parameters.
According to a preferred embodiment of the method for operating the vehicle brake as a hand brake according to the invention, one average value each is formed from several of the speed and/or current consumption reference values stored in the memory in order to compensate for tolerances which can occur due to normal wear as well as due to temperature fluctuations. The averaging can for example be effected over a certain number of actuations, but continuously as well.
In a particularly preferred manner, the method according to the invention is developed further in that the maximum current consumption of the electric motor, representing a certain clamping force, is automatically varied as a function of one or several parameters. For example, the clamping force can, in this manner, be automatically adapted to the vehicle load and/or the inclination or the steepness, respectively, of the roadway, because it is reasonable that with a level roadway or an empty vehicle a lower clamping force for blocking same is required.
A generic vehicle brake, besides its normal function as a hydraulic service brake, however, is capable of being employed not only as a hand brake, but also as an emergency brake. For this purpose, the spindle of the spindle/nut arrangement is driven by the electric motor in a first sense of rotation similarly to the approach in the employment as a hand brake in such a manner that the nut is moved towards the brake piston and eventually contacts the brake piston. The spindle is then further rotationally driven in the first sense of rotation by means of which the brake piston is displaced and the friction member is moved into contact with the rotor to be braked. The vehicle deceleration which is thereby achieved is then compared to a specified minimum deceleration. For this purpose, e.g. the vehicle deceleration which is determined anyway in an anti-slip brake control system (ABS system) which is installed as standard in most road vehicles today, can be utilized, while the specified minimum deceleration can, e.g. be a statutorily specified minimum deceleration. The spindle is then, with the aid of an electronic control unit which can be combined e.g. with the control unit of the brake pressure regulating system, rotationally driven as a function of the result of the previously performed comparison between the momentary vehicle deceleration and the specified minimum deceleration in the first sense of rotation or counter thereto in a controlled manner in such a way that, on the one hand, the value does not drop below the specified minimum deceleration and, on the other hand, the vehicle wheels do not block. In an emergency situation, for example, a failure of the hydraulic actuation of the vehicle brake, a vehicle can be brought to a standstill in this manner with at least the specified minimum deceleration. Such a metered operation of the vehicle brake according to the invention can also be used for facilitating hill climbing in that initially a rolling back of the vehicle is prevented by the already described blocking function of the brake (so-called xe2x80x9chillholderxe2x80x9d function) and that upon starting, the brake is released slowly and controlled in a manner corresponding to the velocity increase of the vehicle.
According to a preferred further development of the previously described method a control means, e.g. the already mentioned control unit, which cooperates with the vehicle brake(s) permits the execution of the method in a vehicle brake only if both the vehicle wheel associated with this vehicle brake as well as the diagonally opposite vehicle wheel are rotating. If two diagonally opposite vehicle wheels are rotating, this means that the vehicle is moving. The previously mentioned control is necessary, for example, to enable a function check of the hand brake in a brake test stand wherein the wheels of one vehicle axle are rotationally driven.
One embodiment of the vehicle brake according to the invention together with a method for operating the vehicle brake will be described in more detail in the following with reference to the attached schematic figures, in which: